Bad posture can be corrected by stretching the shortened tendons and muscles and shortening the lengthened muscles. It's not really a quick process but its not necessarily hard. For rounded shoulders you need to stretch your arms back to lengthen the front of the chest and do strenghthening on your back to shorten those muscles and pull your shoulders back naturally. Strengtheing the core can also improve posture. You need the muscles around your spine to be in tip top shape to pull it into correct alignment. I found a work out video called 10 Minute Solution: Target Toning for Beginners. It's not the most exciting workout, but I've found it to be quick, easy and effective. It specifically targets posture muscles in the ab and upper body sections of the workout. What I really love about it is you can pick and choose what workouts you want to do in the menu section and it will automatically play them.
10 Minute Solution at Amazon: http://www.amazon.com/10-Minute-Solution-Target-Beginners/dp/B0002J58NI
Remember not to do the floor exercises for your tummy if you have a diastisis.